1. Field of the Invention
The present invention is directed to electrical discharge machines (EDMs) and more particularly, to an electrical sparking drill and a method for forming a hole with an electric spark.
2. Description of the Related Art
There are at least two types of EDMs for using an electrode to form a hole in a workpiece; wire cut machines and sparking drill machines. Within the sparking drill machine category, a liquid may be utilized between the electrode and the workpiece. In some conventional sparking drills, oil or kerosene is utilized as the liquid. However, these systems have a high probability of catching fire due to the combustibility of oil and/or kerosene in the presence of the electric spark.
A second type of sparking drill utilizes water as the liquid. Such conventional sparking drills utilize a high voltage, namely on the order of five times the arcing voltage, and high resistivity, on the order of 100,000-1,000,000 cmΩ. However, the high voltage applied wears down the electrode and often wears down the electrode unevenly, both of which are undesirable.
FIG. 1 illustrates a conventional electric sparking drill for drilling one or more holes in a workpiece 10 utilizing an electrode 12. A voltage vg is applied across the electrode 12 and a base 14, supporting the workpiece 10. The applied voltage vg, the gap 16′, and the resistivity of the liquid 18 supplied from tank 20 determine whether arcing occurs between the electrode 12 and the workpiece 10 in order to machine a hole in the workpiece 10. The combination of parameters including at least the voltage of vg, the gap 16′, and the resistivity of the liquid 18 may result in a desirable arcing condition or two undesirable conditions, an open circuit or a short circuit. The liquid 18 is pumped via pump 22 into the gaps 16 and 16′. Ideally, arcing only occurs at the end of the electrode 12 at gap 16′, in order to efficiently machine the hole in workpiece 10. Arcing on the sides of the electrode 12 at gap 16 is undesirable and degrades the efficiency and speed with which the workpiece 10 may be machined. Typically, in prior art systems, the liquid 18 is de-ionized or pure water with a high resistivity in the range of 100,000-1,000,000 cmΩ. The pump 22 conventionally supplies the liquid 18 at a pressure of approximately 50 bars and at a flow rate of 60-100 cc/min. The voltage vg is supplied using a DC current source 24, a switching element 26, a current limiting resistor 28, a pulse generator 30, amplifiers 32 and 34, a mean voltage controller 36, a reference voltage Vr and a feedback voltage Vf. Conventionally, the DC source 24 supplies a voltage on the order of four to six times the arcing voltage Va.
The pump 22 supplies the liquid 18 via a high pressure joint 23. The amplifier 34 supplies a voltage to the motor Mz. The motor Mz controls the position of the electrode in the z axis as illustrated in FIG. 1. The motor Mc controls the speed at which the electrode 12 revolves. The feedback voltage Vf is supplied via a brush contact 38.
When the voltage across the gap Vg reaches a predetermined level, an electric sparking drill or arc is formed across the gap 16′. As a result, the arc passes from the electrode 12 and terminates on the workpiece 10, creating a high temperature explosion at the workpiece 10, thus causing the workpiece 10 surface to decompose. Typically, the surface is melted and dispersed as resolidified chips that are retained in the gap 16′. Due to a pumping action of the electrode 12 caused by a periodic up-and—down “jump” of the electrode 12, the liquid 18 washes most of the chips out of the gap 16′.
FIG. 2 illustrates the operation of the conventional electric sparking drill illustrated in FIG. 1. The peak voltage Vp is essentially equal to the open voltage, that is the voltage necessary to create an open circuit at the gap 16′. The width of the pulses of the peak voltage is controlled in order to supply a consistent mean voltage Vm. The arc voltage Va is the voltage at which a spark occurs at the gap 16′ and the electrode 12 can machine a portion of the workpiece 10 in order to create the desired hole.
FIG. 3 illustrates the electrode 12 and the gaps 16 and 16′ in more detail. As more clearly illustrated in FIG. 3, the liquid 18 travels through a center canal 13 of the electrode 12. The electrode 12, with a voltage applied thereto, and the liquid 18 interact at the gap 16′ to create an arc to form a hole in the workpiece 10. However, if the conductivity, voltage, and gap size are not correctly selected, arcing can also occur at gap 16, which is undesirable. Arcing at the gap 16 causes the electrode 12 to wear, possibly unevenly. Further, as illustrated in FIG. 3, a high open voltage on the order of four to six times the arcing voltage wears the tip of the electrode 12 down, which causes the formation of remaining stock areas 11, caused by electrode 12 wear.
Further, once the electrode 12 goes entirely through the workpiece 10 and makes a hole on the other side, it is impossible to keep the liquid 18 at the gap 16′. Because the hole prevents the liquid 18 from being held in the gap 16′, the spark must be made in air, which makes it very difficult to remove the remaining stock areas 11 and very difficult to keep the electrode 12 moving at high speed.
FIG. 4 illustrates a conventional relationship between the gap 16′ and the arcing voltage Va for peak voltages equal to two, three, four and five times the arcing voltage Va, respectively. A typical arcing voltage Va is 17-20 volts. The curves of the graph of FIG. 4 essentially illustrate the mean voltage applied at the gap 16′. As illustrated in FIGS. 1 and 2, the mean voltage controller 36 is utilized to control the mean voltage applied at the gap 16′. As seen in FIG. 4, voltages near the arcing voltage Va, shown as the area 40, result in conditions which are extremely difficult to control.
FIG. 5 illustrates the difficult to control area 40 in more detail. FIG. 5 illustrates a curve of the working speed W of the electrode 12 and the current Ig as a function of the mean voltage and the gap distance for the arrangement in FIG. 1, where a high voltage is utilized as applied by DC source 24. As is clearly illustrated by the steepness of the W curve near Va in FIG. 5, it is extremely difficult, if not impossible, to control the arrangement illustrated in FIG. 1 at a voltage of approximately the arcing voltage, Va. Due to the steepness of the curve W, it is difficult to determine whether an arcing, open, or short condition exists at the gap 16′ at the arcing voltage Va. As a result, in conventional systems, such as the one illustrated in FIG. 1, a higher voltage is utilized, closer to Vp, such as 2Va, 3Va, 4Va, or 5Va is used.
As described above, in an electric sparking drill operation, the electrode 12 confronts the workpiece 10 to be machined with a gap 16′ therebetween, and an electric discharge is caused in the gap 16′ while a drilling solution, liquid 18, is supplied to the gap 16′, to machine the workpiece 10 as required. If the electric sparking drill apparatus is used to bore a hole in the workpiece 10, a trimming die having a desired configuration can be formed with high accuracy.
As a result, this operation is useful for forming various metal or other molds. However, conventional systems, such as the one illustrated in FIG. 1, have several other deficiencies. First, conventional methods of forming a hole in a workpiece 10 using an electric sparking drill cannot automatically detect the time instant when the end of the electrode 12 penetrates the workpiece 10. Heretofore, in order to detect the penetration of the electrode 12, the amount of feed of the electrode 12 was estimated or an operator was required to watch the movement of the electrode 12, during the drilling operation.
Accordingly, although the hole has been formed in the workpiece 10, the electrode 12 is still supplied with current to continue the electric sparking drill machining operation. As a result, the nominal drilling time is increased, and the side of the trimming die is excessively machined i.e., drilling accuracy is considerably lowered. As described above, in order to detect when the electrode has penetrated the workpiece to thereby end the drilling operation, the operator must carry out troublesome work, which lowers work efficiency and makes it difficult to provide an automatic electric sparking drill.
Prior art electric sparking drill apparatus are further disadvantageous in that, in the case where a through-hole is formed in the workpiece 10 with an electrode 12 which is a fine electrode tube, the electrode tube vibrates depending on the amount of feed of the electrode 12 after the electrode 12 has penetrated the workpiece 10; that is, the electrode tube does not move in a straight line after penetrating the workpiece 10, and as a result, the configuration of the hole thus formed is adversely affected.
Conventional electric sparking drill apparatus are further disadvantageous in that the electrode 12 and the workpiece 10 are usually of opposite polarity. Typically, the electrode 12 is strongly negative and workpiece 10 is strongly positive, which results in the migration of ions from the workpiece 10 to the electrode 12. This erosion has a negative effect on the workpiece 10.
Prior art electric sparking drill apparatus are further disadvantageous in that the electrode 12 is usually very fine, with a very small inner and outer diameter, and is therefore easily damaged or deformed. Using a damaged or deformed electrode 12 results in a poor quality hole in the workpiece 10.